During operation of a gas turbine engine heat is generated in various parts of the engine and its accessories and this heat is transported around by the engine's fluid flow systems.
It is common practice in gas turbine engines to transfer heat from the engines lubrication system into the fuel system by means of a heat exchanger. The lubricating oil picks up heat from bearings, the pumping process and other sources and transfers this heat to the fuel as it passes through the heat exchanger. This heat transfer prevents the oil overheating and raises the fuel temperature prior to combustion.
As gas turbine shaft speeds and cycle temperatures have increased it has become necessary to increase oil flows. These increased oil flows are only required at high engine powers, however because the oil pumps are constant displacement units the oil flows have also increased at low engine powers. The result is that at low powers an increased amount of heat is rejected into the fuel system.
One solution to this problem is to manage the excessive fuel temperatures at low powers by the use of an additional heat exchanger. Air, which bypasses the core engine, is used to cool the fuel either directly or to pre-cool the oil to reduce the fuel temperature. The exact configuration used is determined by the application however both configurations require the use of a valve to turn the airflow on/off to minimise any performance penalty when the extra cooling is not required.
The use of an additional heat exchanger to manage the excessive fuel temperatures involves a significant penalty in terms of engine weight, cost and specific fuel efficiency.